The present invention generally relates to devices for heating liquids, and more particularly to a compact ribbon-flow cartridge-type blood warmer.
Blood is generally stored at a temperature near 4.degree. C. Prior to intravenously injecting stored blood into the human body, it must be heated to near physiologic temperature (32.degree. C. to 37.degree. C.). During massive transfusions flow rates as high as 160 milliliters per minute may be required.
Various kinds of blood warmers presently exist. Most utilize a flexible plastic container or conduit for the blood being heated, such as plastic tubing immersed in a warm water bath or a plastic bag or pouch sandwiched between heating plates. Usually the heated flow path is quite long (in the case of tubing) or the flow is otherwise intentionally restricted (in the pouch-type heaters) to promote uniform heating of the liquid along the heated flow path.
Flexible plastic blood containers or conduits need to have substantial wall thicknesses (for example, a minimum of 0.10 mm or 4 mils is recommended for the Le Boeuf heater, U.S. Pat. No. 4,309,592) in order to prevent both rupture and, in the case of bag-type units, localized wrinkling or folding of the bag between the heating plates which would impede fluid flow. As plastic materials are good insulators but in general poor conductors of heat, the necessity of having relatively thick plastic walls imposes a requirement of large heating areas and/or fairly high heating plate temperatures in order to obtain an adequate rate of heat transfer to the blood. High heating plate temperatures impose a risk of damaging blood by overheating; large heating areas imply large priming volumes with a consequent waste of that volume of blood. Long or restricted flow paths increase the hydraulic resistance to fluid flow, thereby preventing high blood flow rates unless a high head of pressure is available; high pressure, however, tends to become self-defeating as thicker plastic walls are needed to avoid the prospect of rupture. The water bath type of blood warmers tend to underheat or overheat, depending on the rate of blood flow, and additionally are an inconvenience to have occupying space in the operating room.
Consequently a need exists for a reliable, rugged, simple and compact blood warmer, having a small priming volume and low fluid-flow resistance, that is able to warm stored blood to physiologic temperature at from low to high flow rates.